Electric resistance welding process



Patented Jan. 19, 1954 UNITED STATES- PATENT OFFICE ELECTRIC RESISTANCEWELDING PROCESS Albert J. Elleman, London, England, assignor to ImperialChemical Industries Limited, a corporation of Great Britain No Drawing.Application July 24, 1951, Serial No. 238,373

9 Claims. ((1219-10) This invention relates to a process of electricresistance welding metal parts which have been parts prior to weldingwith a paint or lacquer,

e. g. an anticorrosive primer, but it is found that the film of coatingmaterial seriouslyinterferes. with the electric resistance weldingprocess. This is due to the fact that the film of coating material has ahigh electrical resistance and in fact, at the low voltages used in thewelding process, is a very good insulator. As a, result no current canbe passed through the metal parts and so no heat is developed. It isknown to reduce the electrical resistance of coating compositions byadding a conductive material such as powdered zinc, copper, silver orcarbon black but the high proportion of these materials required to givethe necessary low electrical resistance results in the coatingcomposition having very poor protective properties. In practice the onlysatisfactory solution of this welding problem has been to remove thecoating film in the area to be welded prior to pressing the partstogether.

We have now found that certain types of coat ing'material have anelectrical resistance low enough. to permit electricresistancdwelding'of metal parts coated with such material. The

present invention provides a proces's of electric resistance weldingmetal parts which have been coated with a paint or lacquer containing asubstantially non-oxidised magnetic metal powder. In a preferred form ofthe invention-the metal powder is one possessing magnetic remanence.

The advantage to be derived from'the invention may be illustrated asfollows:

A standard zinc chromate anticorrosive primer Was modified by theaddition of (i) Zinc powder (ii) Brasspowder (iii) Stainless steelpowder (iv) Soft iron powder (0) Nickel powder the proportion of metalpowder being in each case 13.8% by volume of the solids of the primer.The powder was thoroughly stirred into the primer, metal panels werecoated with it and the electrical resistance of the dried film wasmeasured between electrodes 1 in diameter. The results were:

Table I Film resistance,

Film composition mimohmS/thou Unmodified primer 50, 000 Prlmer+zinc 25,000 Primer+brass l 25, 000 Primer+stainless steel. 30, 000 Primer-l-softiron 3, 000 Primer+nickel 30 The term thou is a measurement of thethickness of the paint film, and is an abbreviation of a thousandth ofan inch.

It is believed that the relatively very low resistances obtained whensoft iron or nickel are present in the paint film is due to the factthat the metal particles, by reason of their inherent magneticproperties, link together to form conductive chains in the paint film.The difference between the soft iron and nickel figures may be due tothe presence on the soft iron particles of a thin layer of oxide. Thatthis will afiect the conductivity of a chain of the particles may beshown by measuring the resistance of the powders after being compactedunder pressure in tubes.- Whereas the resistances of soft iron andnickel in bulk are almost identical, the compacted iron powder had arei'stance of 1500 ohms/cm. and the compacted nickel powder had aresistance of 0.03 ohms/cmS. @The conductivity of a paint filmcontaining soft iron powder may be increased by reducing any oxide filmon the powder before incorporating it in the paint. Other magneticmetals suitable for the purpose of this invention are steel and alloyssuch as Permalloy and Alnico.

Mere stirring in of the metal powder, although giving a usefulindication of the properties'of the composition, is not sufiicient toproness in the paint film. it is necessary'to ball- Table IIMagnetisingficld in genes Percent vol. nickel in dry film Theresistances of the films are expressed in milliohms/thou.

The improvement is most marked with metals such as nickel which possessthe property of magnetic remanence and in general it has been found thatthe higher the degree of magnetic remanence the more suitable is themetal powder for the purpose of this invention. However even magneticmetals with zero remanence give films much more zconductive than thosecontaining a non-magnetic metal. 'For example a paint film containing anickel-iron alloy of zero remanence had an electrical resistivity of2,500 .millie ohms/thou, i. e. only one tenth of the resistance of asimilar paint film con ain n z n powder- The effect of varying theproportion of magnetic. metal powder also is shown in Table II, and infactthe paint film may contain up to 30% by volume of magnetic metalwithout its protective properties being adversely afiected.

Experiment has shown that in the ease of straight time spot weldingsatisfactory welds canhe obtained if the film of paint or lacquer hasa rsistance... a m asu e over thcarc 9i the electrodes, of, not more than,60 Qh'lfllmilliohmsAthou, the thickness, of the film being from; 11-2thou. Table II indicates that in the case of the primer modifiedwithnickel powder satisfactory welds, will beobtained if the primer containsas little as 7.5% nickel by volume of the. pr mer olid Tre tment at: theincd fledp mcr witha magnetic field makes pps ihle eu e ore smallerproportion of nickel: powder. In-th case. ofw potent-ial time spotwelding satisfactory welds can be obtained if the film ofpaintorlacquer-has a resistancaas measured over-the area of the electrodes,of as much as 5,000 milliohms/thouf Even in this case however, Table Iindicates; that the primer modified with zinc powder would beunsuitable.

The conductivity is also influenced by ,the size and shape of theparticles. of the powdered metal and/{have found that. the lower theapparent density of the powder, the lower is the electrical e t vi r 9 emin fi m re. and; the. weightof t Sample and 4 then being measured. Theeffect of this factor is illustrated in Table III.

Table JII Coating composi Coating composl- Coating composi- A t tion Ition II tlon III pparen density nickel Vol. I Vol. Vol. powder 1 percentMllh' percent; m percent Ni in Ohms/ -;Ni,ln" Ohms/ Ni m hmsl dry filmthou dry'film thou dry film thou The volume Percentage of nickel in thedry film was calculated on the real density 8.9 of :nickel, and ingeneral, I have found that for the purpose of this invention magneticmetal powders with a ratio of real density to apparent density (asmeasured above) of greater than 4:1 give better results.

, Experiments carried out on welds made on plain steel and welds madeaccording to the process of this invention showed no significant changeseither in the welding process or in 'the'shear strength of the welds.Oscilloscope traces of the welding current showed no variation in char-:acter between welding the ,plain and painted steeland shear strengthtests on the welds gave 7 the following results;

N These indicate that the paint film has ,no

' significant effect on the-strength of :the weld.

What I claim is: 1. A. process for the electric resistance welding ofprotectively precoated metal parts in which the'coating is dry whichcomprises reducing the electrical resistance of thecoating byincorporating in intimate admixture in the coating 10pm: position aon-oxidized magnetic metal powder, pressing the said coated metal parts,togetherzto provide a contact areaand passing :an electric reatdensity? r he ci x em a as: u d" current through the area of contact.

2. Aprocess as recited in claim 1 in which the magnetic metal powderpossesses a 10W apparent vcwsl cns y- A prc s ea -da ed inc aimllawman-the al; ;;p0wder,- possesses :rnegnetic .4, A process as recitedin claim .1 i .whichzth magnetic metal-powder ;is present in the-coatingcomposition in a substantial amount not ex-. ceecling 30%. v

5. A process as recited in claim 4 in whichthe magnetic metal powderpossesses magnetic remanence.

6. A process as recited in claim 1 in which the magneticmetal powder issoft iron.

7. A process as recited in claim 1 in which the magnetic metal powder isnickel.

8. A process for the electric resistance welding of protectivelyprecoated metal parts in which the coatingr-isdry which comprisescoating the metal parts with aprotective coating'composition containingin intimate admixture anon-oxidized magnetic metal powder, drying thecoating,

pressing the dry coated metal parts together to provide a contact areaand passing an electric current through the area of contact.

9. A process as recited in claim 8 in which the a coating composition issubjected to a magnetic field prior to the drying of the coatingcomposition.

ALBERT J. ELLEMAN.

6 References Cited in the file of this patent UNITED STATES PATENTSNumber 2,375,178 2,510,727

Number Great Britain Jan. 8, 1931

1. A PROCESS FOR THE ELECTRIC RESISTANCE WELDING OF PROTECTIVELYPRECOATED METAL PARTS IN WHICH THE COATING IS DRY WHICH COMPRISESREDUCING THE ELECTRICAL RESISTANCE OF THE COATING BY INCORPORATING ININTIMATE ADMIXTURE IN THE COATING COMPOSITION A NON-OXIDIZED MAGNETICMETAL POWDER, PRESSING THE SAID COATED METAL PARTS TOGETHER TO PROVIDE ACONTACT AREA AND PASSING AN ELECTRIC CURRENT THROUGH THE AREA OFCONTACT.